Twin-tube hydraulic damper assemblies in motor vehicle suspension environments are subject to many different types of stress and highly variable amplitudes and frequencies of compression and rebound. High frequency vibrations of the piston rod inside a typical twin-tube assembly can result in rattling, rambling, and knocking noises and when the damper is a part of a vehicle suspension system, these vibrations may be audible even inside the passenger compartment. The rattling noise intensity and occurrence depends on the particular vehicle suspension setup and the acoustic performance of the vehicle chassis. Nevertheless, these noises occur mainly at low amplitude, high frequency damper stroke reversals which occur, for example, while driving over a road paved with setts, also called Belgian blocks, with relatively low speeds of around 12 to 25 miles per hour (20-40 km/h). The most irritating rattling noise occurs at a piston rod vibration frequency within the range of 300-600 Hz. Poor isolation parameters of the vehicle chassis and a top mount at the end of the piston rod, as well as a low level of a background noise related to the low vehicle speeds at which this rattling noise occurs additionally makes it hard for vehicle drivers and passengers to ignore
The present inventors have discovered that these vibrations are generated by a deflective disc of a rebound base valve assembly due to pressure fluctuations occurring, in particular, during rapid changes of the stroke direction of the piston.
Therefore, it is an object of the present invention to provide a twin-tube hydraulic damper in which these vibrations would be significantly suppressed and consequently the rattling noise they produce would be significantly decreased. It is also desirable to accomplish this object in a cost efficient and simple to manufacture method.
The prior art discloses other solutions providing improvements of acoustic performance of hydraulic dampers. Publication US2012292147, for example, discloses a twin tube damping mechanism having a piston body provided with a base valve assembly having an additional deflective disc named a velocity sensitive valve separated by a spacer from the main deflective disc. This velocity sensitive valve disc defines a plurality of slots or apertures which allow the flow of fluid when the velocity sensitive valve disc is in its closed position. Various kinds of other noise and vibration suppressing arrangements provided on damper piston assemblies are disclosed, for example, in publications U.S. Pat. No. 5,813,500, DE2727407, GB2347728, US2015247545, DE102008042251 and JPH10220515.